1. Field of the invention
The present invention relates to a method and an apparatus for driving a capacitive flat matrix display panel such as a thin film EL display device and/or plasma display.
2. Description of the prior art
A double insulation type (or 3-layer structure) thin film EL element, for example, is formed in the following way.
As shown in FIG. 1, strips of transparent electrodes 2 made of In.sub.2 O.sub.3 are provided, in parallel, on a glass substrate 1. Over these transparent electrodes 2 are formed a layer of dielectric materials, 3a such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3 and an layer of EL material 4, made of ZnS which is doped with an activation agent such as Mn. Further, a layer of dielectric material 3b is included, such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 and Al.sub.2 O.sub.3 similar to the above layer 3a, by means of thin film forming technique such as vacuum evaporation or sputtering. Thus, a 3-layer structure is created, each of thickness from 500 to 10000 .ANG.. Over this 3-layer structure is provided parallel strips of back electrodes 5 made of aluminum arranged at right angles to the transparent electrodes 2.
Because the thin film EL element is formed by interposing an EL material 4 which is sandwiched by dielectric material layers 3a and 3b between electrodes, it can be regarded as a capacitive element in terms of an equivalent circuit. As is apparent from the voltage-brightness characteristic shown in FIG. 2, the thin film EL element is driven by applying a relatively high voltage of about 200 V.
Basic display operation of a thin film EL display device which uses the thin film EL element as the display panel is driven, while using the transparent electrodes 2 of the thin film EL element as the data electrodes and using the back electrodes 5 as the scanning electrodes, by applying a modulation voltage, corresponding to the display data which determines whether to illuminate display elements or not, to the data electrodes and applying writing voltage to the scanning electrodes in the order of the lines. With this display drive method, superposition or cancellation between the writing voltage and the modulation voltage occurs in the part of picture elements of the EL layer where the scanning electrodes and the data electrodes intersect, resulting in a voltage higher than the luminescence threshold voltage or below the luminescence threshold voltage applied to the picture element. This thereby makes each picture element illuminate or not, respectively, to achieve the specified display pattern.
A popular method of driving such a thin film EL display device of the prior art, which is adopted to provide gradation display by changing the brightness of each picture element in multiple levels, is the pulse width modulation method. In this method, pulse duration of the modulation voltage applied to the data electrode is changed in accordance to the gradation display data (brightness data) to thereby control the area (intensity) of the voltage applied to the picture element.
FIG. 3 (1), (2) and (3) show the waveforms of a modulation voltage V.sub.M applied to the data electrode, a writing voltage V.sub.W applied to the scanning electrode and a voltage V.sub.A applied to the picture element in the pulse width modulation method.
In this case, a symmetrical driving method as described below is generally adopted since it is capable of maintaining display quality of a thin film EL element which is driven with alternate current; the symmetrical driving method completes one frame of display with an N-driven field which uses a voltage -V.sub.NW having a level corresponding to the luminescence threshold voltage -V.sub.th and of a reverse polarity to the modulation voltage V.sub.M ; and a P-driven field which uses a writing voltage V.sub.PW of a level, which corresponds to the sum (V.sub.th +V.sub.M) of the luminescence threshold voltage V.sub.th and the modulation voltage V.sub.M, having the same polarity as the modulation voltage V.sub.M.
Either with the N driving or with the P driving, voltage V.sub.A which is applied to the data electrode is given as the difference of the potential V.sub.X of the data electrode and the potential V.sub.Y of the scanning electrode V.sub.X -V.sub.Y. In the N driving, voltage V.sub.A of a waveform, which is the modulation voltage V.sub.M superimposed on the absolute value V.sub.NW of the writing voltage -V.sub.NW, is applied to the picture element as shown in FIG. 3 (3). Therefore, the voltage beyond the luminescence threshold voltage V.sub.th is applied in the section where the modulation voltage V.sub.M and the writing voltage V.sub.PW are superimposed. That is, area (intensity) of the voltage V.sub.A applied to the picture element in the N driving increases as the pulse duration of the modulation voltage V.sub.M elongates, and decreases as the pulse duration becomes shorter, as shown by the dashed lines in FIG. 3 (1) and (2).
On the other hand, in the P driving, voltage V.sub.A of a waveform which is the writing voltage V.sub.PW subtracted by the modulation voltage V.sub.M is applied to the picture element. Therefore a voltage beyond the luminescence threshold voltage V.sub.th is applied in the section where the modulation voltage V.sub.M and the writing voltage V.sub.PW are not superimposed. That is, in P driving, the area (intensity) of the voltage V.sub.A applied to the picture element increases as the pulse duration of the modulation voltage V.sub.M becomes shorter, as shown by the dashed lines in FIG. 3 (1) and (2).
As described above, either in the N driving or the P driving, gradation display is performed by variably setting the pulse duration of the modulation voltage V.sub.M in accordance to the gradation to be displayed (voltage changes in opposite senses in N driving and P driving).
However, the above driving method has been suffering a problem of unstable gradation brightness and being unable to set many gradation levels, as described in the following.
FIG. 4 (1), (2) and (3) show, for the purpose of explaining the cause of the problem, the voltage waveform applied to the picture element, the waveform of the accompanying current of the power source and the waveform of current flowing through the luminescent layer of the picture element, respectively, in the conventional pulse width modulation method.
Voltage V.sub.A shown in FIG. 4 (1) corresponds to the waveform shown in FIG. 3 (3). When Voltage V.sub.A of square waveform is applied to the picture element, current of the power source takes the waveform as shown in FIG. 4 (2).
That is, the current flowing, before the Voltage V.sub.A reaches the luminescence threshold voltage V.sub.th nearly a constant current which flows through the capacitive component of the picture element and does not contribute to the luminescence. When the Voltage V.sub.A reaches the luminescence threshold voltage V.sub.th, the current component which flows through the luminescent layer of the picture element, or the current component which contributes to the luminescence, is added to that flowing through the capacitive component of the picture element. This results in the current flowing through the luminescent layer being that shown in FIG. 4 (3). Illumination brightness of the picture element increases in proportion to the current flowing through the luminescent layer.
When the pulse duration of the modulation voltage V.sub.M is limited as shown by the dashed line in FIG. 4 (1), the current flowing through the luminescent layer is shut off when the modulation voltage V.sub.M drops. Thus the current flowing through the luminescent layer of the picture element is controlled by controlling the pulse duration of the modulation voltage V.sub.M, and a brightness corresponding to the pulse duration of the modulation voltage V.sub.M can be obtained.
However, as described above, in the case where the writing voltages -V.sub.NW and V.sub.PW are applied to the picture element in square waveforms, current flowing through the luminescent layer forms a peaked shape as shown in FIG. 4 (3). This leads to a shorter energization period (denoted by t1 in FIG. 4 (1)) and makes it impossible to set the modulation voltage V.sub.M in multiple levels and to control the brightness in multiple levels. In addition, because larger current flows through the luminescent layer in each brightness level, a slight deviation in the pulse duration of the modulation voltage V.sub.M causes a significant variation in the brightness, thus making it difficult to stabilize the brightness gradation.